This disclosure relates generally to power systems, and especially relates to the storage and recovery of energy from a renewable power source and electrochemical cells.
Geographically remote areas such as islands or mountainous regions are often not connected to main utility electrical grids due to the cost of installing and maintaining the necessary transmission lines to carry the electricity. Even in remote communities where the transmission lines are in place, it is not uncommon for frequent and extended power outages due to weather related faults. In either case, to prevent economic loss in times of an electrical outage, it is often necessary for these communities or industries in these regions to create local “micro” electrical grids to ensure a reliable and uninterruptible power system. This uninterruptible power system may be either a primary system where there is no connection to the main utility grid, or a backup system that activates when an outage occurs.
Electrical power for the local grids comes from a variety of sources including hydrocarbon based and renewable power sources. Within a particular grid it is not uncommon to have multiple generation sources, such as diesel generators, natural gas generators, photovoltaic arrays, and wind turbines working in combination to meet the needs of the grid.
Electrical demands placed on the local grid will vary during the course of a day, week, or season. Since it is not often practical or possible to turn generation sources on and off, inevitably excess power will be generated. This excess energy is typically converted into another form of energy such as heat for storage in another medium such as water. In cold climates, the heated water can then be used for other purposes such as heating buildings, cooking or maintaining temperature in equipment. As the load requirements of the grid increase, it is difficult or impossible to recapture the converted energy back into electrical energy for use in the electrical grid. Further complicating matters is that renewable power sources do not typically run continuously at full power and will experience extended periods of low to no energy output (e.g. night time or seasonal low wind periods).
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells commonly employed to address back-up power requirement when a grid fails or when a renewable energy source is unavailable. An electrolysis cell typically generates hydrogen by the electrolytic decomposition of water to produce hydrogen and oxygen gases, whereas in a fuel cell hydrogen typically reacts with oxygen to generate electricity. In a typical fuel cell, hydrogen gas and reactant water are introduced to a hydrogen electrode (anode), while oxygen gas is introduced to an oxygen electrode (cathode). The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, methanol or other hydrogen source. Hydrogen gas electrochemically reacts at the anode to produce hydrogen ions (protons) and electrons, wherein the electrons flow from the anode through an electrically connected external load, and the protons migrate through a membrane to the cathode. At the cathode, the protons and electrons react with the oxygen gas to form resultant water, which additionally includes any reactant water dragged through the membrane to cathode. The electrical potential across the anode and the cathode can be exploited to power an external load.
This same configuration is conventionally employed for electrolysis cells. In a typical anode feed water electrolysis cell, process water is fed into a cell on the side of the oxygen electrode (in an electrolytic cell, the anode) to form oxygen gas, electrons, and protons. The electrolytic reaction is facilitated by the positive terminal of a power source electrically connected to the anode and the negative terminal of the power source connected to a hydrogen electrode (in an electrolytic cell, the cathode). The oxygen gas and a portion of the process water exit the cell, while protons and water migrate across the proton exchange membrane to the cathode where hydrogen gas is formed. The hydrogen gas generated may then be stored for later use by an electrochemical cell.
In certain arrangements, the electrochemical cells can be employed to both convert electricity into hydrogen, and hydrogen back into electricity as needed. Such systems are commonly referred to as regenerative fuel cell systems. Regenerative fuel cells may be used in power generation systems as either primary or secondary power sources. However, because regenerative fuel cell systems generally take a certain amount of time from the point of initial activation to delivering full power, there may be a brief delay of power attendant thereto when switching over from a primary power supply to backup power generated by a fuel cell supply. What is needed in the art is a cost effective apparatus and method for bridging short duration power interruptions.